The present invention relates to a circuit for controlling the brightness of two light sources in opposite directions particularly for purposes of obtaining a gradual transition from one source to the other; the light sources being, for example, included in a slide projector or pertaining to two slide projectors.
Control circuits for slide projectors are known which include means for adjusting the electric current through the projection lamp and in response to electric control signals. The control circuit may, for example, include a potentiometer whose tapped output is converted into a dc current which in turn is fed through a comparator receiving also a sawtooth or ramp signal. The comparator produces output signals which are pulses whose phase depends on the potentiometer voltage. These pulses control electronic circuit elements such as triacs which in turn control the lamps current.
The potentiometer voltage is used directly to control a frequency variable oscillator being in turn connected to a Schmitt trigger with integrating output. The latter is a dc current which directly represents the adjustment of the potentiometer. Upon using two comparators the circuit can readily be used for the control of two lamps and to effect a gradual transition from one to the other. This circuit as described is rather expensive and includes many expensive electronic components. The transition itself results from manual operation of the potentiometer slide unless one includes a motor drive for just that purpose. Therefore, the regularity and smoothness of the transition depends on the evenness of the manual changeover.